1. Field of the Invention
The present invention relates to a holding apparatus for holding an optical element, a telescope including the holding apparatus, and an optical apparatus including the holding apparatus.
2. Description of the Related Art
Heretofore, mechanisms for holding an optical element in a telescope have been known in the art and one of them is disclosed in Japanese Patent Publication No. 05-036766. According to Japanese Patent Publication No. 05-036766, a support made of crystalline plastic, a material having a higher thermal expansion coefficient than that of a metal is arranged between a reflector (one of optical devices) and a barrel made of metal.
In other words, a crystalline plastic material is placed between the reflector and the barrel at ordinary temperature. At higher temperatures, the diameter of the barrel made of a material with a high thermal expansion coefficient becomes higher than the diameter of the reflector. As a result, a difference between the outer diameter of the reflector and the inner diameter of the barrel is caused and a gap is consequently formed between these members. In Japanese Patent Publication No. 05-036766, a holding mechanism is designed so that the gap will be plugged with a crystalline plastic material with a higher thermal expansion coefficient than that of the barrel.
In addition, Japanese Patent Publication No. 05-036766 describes that “even in a state that a rapid decrease in surrounding temperature of a main reflector 3 occurs and a support 9 is pressed against the outer surface of the main reflector 3, the support 9 tends to be deformed by distortion to prevent the main reflector 3 from causing strain deformation and damages”. In general, the optical system of a telescope is constructed of a plurality of optical elements including reflectors, lenses, and so on. However, it is well known that aberration, or so-called decentration aberration, occurs to cause a decrease in image quality (optical performance) when the light axes of the respective optical elements are not aligned in line. In Japanese Patent Publication No. 05-036766, there is a description that a reflector-holding member is “deformable”, so that it may tend to cause decentration aberration when the reflector-holding member is under load of a lens.
In particular, when observing an astronomical object near the horizon, the optical axis of a telescope is substantially horizontal (parallel to the ground) and the weight of a lens in the telescope is supported by the lower part of a barrel via the periphery of the lens. If the holding member located at the lower part is made of a soft plastic material or the like, the holding member may be deformed and decenter the lens held by the holding member downward. The weight of the lens becomes heavier as the aperture of the lens becomes larger. Therefore, the use of a deformable material for a lens-supporting portion may cause a considerable deformation of such a portion and, as a result, a decrease in optical performance of the optical system due to the decentring of the lens.